Allow editing all Stores through the admin

[jhawthorn]

Currently the "General" settings page is mostly (entirely?) for editing the default Store. It should be replaced with an interface for editing any of the stores in the DB.

This would probably fix some awkwardness we see like #1176

[jasonfb]

Are multiple store supported natively in core? I thought support for this was provided though the solidus_multi_domain gem. It would seem logical then that it is that extension's responsibility to provide this.

[jhawthorn]

solidus_multi_domain includes some specifics about what multi-store behaviour means, but to some degree Stores have been natively supported since spree 2.3 (models in core, current_store method, association to orders) https://github.com/solidusio/solidus/blob/master/core/app/models/spree/store.rb